NSW Seasonal Rainfall Outlook: probabilities for April to June 2006,
issued 23rd March 2006

50:50 chances for above average June quarter rainfall

The current seasonal rainfall odds do not strongly favour either wetter or drier
than average conditions. Therefore, the chances of accumulating at least median rain during the June
quarter (Apr-June) are close to 50% across New South Wales, the Bureau of Meteorology
announced today.

The outlooks are based on relationships between Pacific and Indian Ocean
temperatures and Australian rainfall. Recent ocean temperatures have not been
sufficiently warmer or cooler than average to produce a strong bias in rainfall outlook odds.

For the April to June period, the chances of above median rainfall are
mainly between 45 and 50% across NSW, apart from the far southwest of the State
where they're a little below 45% (see map).

So in years with ocean patterns like the current, about five June quarters
out of ten are expected to be drier than the median over NSW,
with about five out of ten being wetter.

Outlook confidence is related to how consistently the Pacific and Indian
Oceans affect Australian rainfall. During the June quarter, history shows this effect
to be moderately consistent along the coast and ranges from Wollongong to the
Queensland border, in the northern border regions around Walgett and in the
far southwest of the State near the Victorian and SA borders. Elsewhere the
effect is only weakly or very weakly consistent (see background information).

After rising to +13 in January, the Southern Oscillation Index (SOI) dropped to a
reading of zero in February. The approximate SOI for the 30 days ending 20th March
was +11. For more information on the SOI and the current status of ENSO, please see
the ENSO Wrap-Up.

Click on the map above for a larger version of the map. Use the reload/refresh button to ensure the latest forecast map is displayed.

More information on this outlook is available from
9.00am to 5.00pm (EST) Monday to Friday by
contacting the Climate and Consultancy section in the
Bureau's Sydney Office: (02) 9296 1522.

Background Information

The Bureau's seasonal outlooks are general statements about the probability
or risk of wetter or drier than average weather over a three-month period.
The outlooks are based on the statistics of chance (the odds)
taken from Australian rainfall/temperatures
and sea surface temperature records for the tropical Pacific and Indian Oceans.
They are not, however, categorical predictions about future rainfall,
and they are not about rainfall within individual months
of the three-month outlook period.
The temperature outlooks are for the average maximum and minimum temperatures
for the entire three-month outlook period.
Information about whether individual days or weeks may be
unusually hot or cold, is unavailable.

Probability outlooks should not be used as if they were categorical forecasts.
More on probabilities is contained in the booklet
The Seasonal Climate Outlook - What it is and how to use it,
available from the National Climate Centre.
These outlooks should be used as a tool in risk management
and decision making.
The benefits accrue from long-term use, say over 10 years.
At any given time, the probabilities may seem inaccurate,
but taken over several years, the advantages of taking account
of the risks should outweigh the disadvantages.
For more information on the use of probabilities,
farmers could contact their local departments of agriculture or primary industry.

Model Consistency and Outlook Confidence:
Strong consistency means that tests of the model on historical
data show a high correlation between the most likely
outlook category (above/below median)
and the verifying observation (above/below median). In this
situation relatively high confidence can be placed in the
outlook probabilities.
Low consistency means the historical relationship, and
therefore outlook confidence, is weak.
In the places and seasons where the outlooks are most skilful,
the category of the eventual outcome (above or below median)
is consistent with the category favoured in the outlook
about 75% of the time.
In the least skilful areas,
the outlooks perform no better than random chance or guessing.
The rainfall outlooks perform best in eastern and
northern Australia between July and January,
but are less useful in autumn and in the west of the continent.
The skill at predicting seasonal maximum temperature peaks in
early winter and drops off marginally during the second half of the year.
The lowest point in skill occurs in early autumn.
The skill at predicting seasonal minimum temperature peaks in
late autumn and again in mid-spring.
There are also two distinct periods when the skill is lowest
- namely late summer and mid-winter.
However, it must always be remembered that the outlooks
are statements of chance or risk.
For example, if you were told there was a 50:50 chance
of a horse winning a race but it ran second,
the original assessment of a 50:50 chance could still have been correct.

The Southern Oscillation Index (SOI) is calculated using the
barometric pressure difference between Tahiti and Darwin.
The SOI is one indicator of the stage of El Niño or La Niña
events in the tropical Pacific Ocean.
It is best considered in conjunction with sea-surface temperatures,
which form the basis of the outlooks.
A moderate to strongly negative SOI (persistently below 10) is
usually characteristic of El Niño, which is often associated with below
average rainfall over eastern Australia, and a weaker than normal monsoon
in the north. A moderate to strongly positive SOI (persistently above +10)
is usually characteristic of La Niña, which is often associated with
above average rainfall over parts of tropical and eastern Australia,
and an earlier than normal start to the northern monsoon season.
The Australian impacts of 23 El Niño events since 1900 are summarized
on the Bureau's web site
(www.bom.gov.au/climate/enso/).